Engineer’s Scale

An engineer’s scale is a tool for measuring distances and transferring measurements at a fixed ratio of length. It is commonly made of plastic and is just over 12 inches (305 mm) long, but with only 12 inches of markings, leaving the ends unmarked so that the first and last measuring ticks do not wear off. It is used in making engineering drawings, commonly called blueprints, in scale. For example, “one-tenth size” would appear on a drawing to indicate a part larger than the paper itself. It is not to be used to measure machined parts to see if they meet specifications.

In scientific and engineering terminology, a device to measure linear distance and create proportional linear measurements is called a scale. A device for drawing straight lines is a ruler. In common usage both are referred to as a ruler.

Boxed set of 1850s ivory engineer’s scales presented to the railway civil engineer George Turnbull in India. 16 scales are engraved.

In Canada and the United States, this scale is divided into decimalized fractions of an inch, but has a cross-section like an equilateral triangle, which enables the scale to have six edges indexed for measurement. One edge is divided into tenths of an inch, and the subsequent ones are directly marked for twentieths, thirtieths, fortieths, fiftieths, and finally sixtieths of an inch.

The engineer’s scale came into existence when machining parts required a greater precision than the usual, binary fractionalization of the inch as in the architect’s scale for houses and furniture. They were used, for example, in laying out printed circuit boards with the spacing of leads from integrated circuits as one-tenth of an inch.